In order to prevent underground basements or cellars from flooding, it is the common practice in many areas of the United States to provide a sump in the floor of the basement and to position an AC motor driven pump in the sump. Some type of liquid level sensing device is provided to energize the motor when the liquid level in the sump reaches a predetermined height and to deenergize the motor when the level has dropped to a safe level. Various types of sensing devices have been used for this purpose including float operated switches, pressure responsive switches, and conductive and capacitive probes.
The systems of this general type have two basic disadvantages, i.e., they may not function when needed in an emergency because of a power failure or they may fail to operate because the level sensing device fails to operate. In order to avoid the disadvantages of the float operated and pressure operated level sensors, attempts have been made to use conductive or capacitive sensors. However, the level sensing devices of these types may be spuriously actuated by soap suds in the sump. Moreover, the electric sensing probes in these devices are susceptible to corrosion and/or errosion and thus have a relatively short life.
In an attempt to solve the problem of power failure during those periods when pumping is necessary, battery operated DC motor driven pumps have been mounted in the sumps alongside or above the AC motor driven pumps with the DC motor circuit being activated in response to a power line failure. Moreover, alarms have been provided for giving a warning if the battery voltage falls below a predetermined level. Unfortunately, these auxiliary DC systems have left much to be desired in preventing flooding due to AC motor driven pump failure.
By way of example, such systems do not make use of the battery operated pump in those cases where there is no power failure but where the AC driven pump fails, nor is the battery operated pump utilized when the capacity of the AC driven pump makes it incapable of handling the inflow of water to the sump. Also, since the battery operated pumps are rarely used, they sometimes are inoperative when their use is required. This may occur because of the rotor becoming locked up or because the battery is incapable of supplying the power necessary to drive the pump. Although the static battery voltage may appear to be satisfactory, the initial current drain may reduce the battery voltage below the usable value.